package com.maxflow;
import java.util.Random;

public final class StdRandom {

	private static Random random; // pseudo-random number generator
	private static long seed; // pseudo-random number generator seed

	// static initializer
	static {
		// this is how the seed was set in Java 1.4
		seed = System.currentTimeMillis();
		random = new Random(seed);
	}

	// don't instantiate
	private StdRandom() {
	}

	/**
	 * Set the seed of the psedurandom number generator.
	 */
	public static void setSeed(long s) {
		seed = s;
		random = new Random(seed);
	}

	/**
	 * Get the seed of the psedurandom number generator.
	 */
	public static long getSeed() {
		return seed;
	}

	/**
	 * Return real number uniformly in [0, 1).
	 */
	public static double uniform() {
		return random.nextDouble();
	}

	/**
	 * Return an integer uniformly between 0 (inclusive) and N (exclusive).
	 */
	public static int uniform(int N) {
		return random.nextInt(N);
	}

	// /////////////////////////////////////////////////////////////////////////
	// STATIC METHODS BELOW RELY ON JAVA.UTIL.RANDOM ONLY INDIRECTLY VIA
	// THE STATIC METHODS ABOVE.
	// /////////////////////////////////////////////////////////////////////////

	/**
	 * Return real number uniformly in [0, 1).
	 */
	public static double random() {
		return uniform();
	}

	/**
	 * Return int uniformly in [a, b).
	 */
	public static int uniform(int a, int b) {
		return a + uniform(b - a);
	}

	/**
	 * Return real number uniformly in [a, b).
	 */
	public static double uniform(double a, double b) {
		return a + uniform() * (b - a);
	}

	/**
	 * Return a boolean, which is true with probability p, and false otherwise.
	 */
	public static boolean bernoulli(double p) {
		return uniform() < p;
	}

	/**
	 * Return a boolean, which is true with probability .5, and false otherwise.
	 */
	public static boolean bernoulli() {
		return bernoulli(0.5);
	}

	/**
	 * Return a real number with a standard Gaussian distribution.
	 */
	public static double gaussian() {
		// use the polar form of the Box-Muller transform
		double r, x, y;
		do {
			x = uniform(-1.0, 1.0);
			y = uniform(-1.0, 1.0);
			r = x * x + y * y;
		} while (r >= 1 || r == 0);
		return x * Math.sqrt(-2 * Math.log(r) / r);

		// Remark: y * Math.sqrt(-2 * Math.log(r) / r)
		// is an independent random gaussian
	}

	/**
	 * Return a real number from a gaussian distribution with given mean and
	 * stddev
	 */
	public static double gaussian(double mean, double stddev) {
		return mean + stddev * gaussian();
	}

	/**
	 * Return an integer with a geometric distribution with mean 1/p.
	 */
	public static int geometric(double p) {
		// using algorithm given by Knuth
		return (int) Math.ceil(Math.log(uniform()) / Math.log(1.0 - p));
	}

	/**
	 * Return an integer with a Poisson distribution with mean lambda.
	 */
	public static int poisson(double lambda) {
		// using algorithm given by Knuth
		// see http://en.wikipedia.org/wiki/Poisson_distribution
		int k = 0;
		double p = 1.0;
		double L = Math.exp(-lambda);
		do {
			k++;
			p *= uniform();
		} while (p >= L);
		return k - 1;
	}

	/**
	 * Return a real number with a Pareto distribution with parameter alpha.
	 */
	public static double pareto(double alpha) {
		return Math.pow(1 - uniform(), -1.0 / alpha) - 1.0;
	}

	/**
	 * Return a real number with a Cauchy distribution.
	 */
	public static double cauchy() {
		return Math.tan(Math.PI * (uniform() - 0.5));
	}

	/**
	 * Return a number from a discrete distribution: i with probability a[i].
	 * Precondition: array entries are nonnegative and their sum (very nearly)
	 * equals 1.0.
	 */
	public static int discrete(double[] a) {
		double EPSILON = 1E-14;
		double sum = 0.0;
		for (int i = 0; i < a.length; i++) {
			if (a[i] < 0.0)
				throw new IllegalArgumentException("array entry " + i
						+ " is negative: " + a[i]);
			sum = sum + a[i];
		}
		if (sum > 1.0 + EPSILON || sum < 1.0 - EPSILON)
			throw new IllegalArgumentException(
					"sum of array entries not equal to one: " + sum);

		// the for loop may not return a value when both r is (nearly) 1.0 and
		// when the
		// cumulative sum is less than 1.0 (as a result of floating-point
		// roundoff error)
		while (true) {
			double r = uniform();
			sum = 0.0;
			for (int i = 0; i < a.length; i++) {
				sum = sum + a[i];
				if (sum > r)
					return i;
			}
		}
	}

	/**
	 * Return a real number from an exponential distribution with rate lambda.
	 */
	public static double exp(double lambda) {
		return -Math.log(1 - uniform()) / lambda;
	}

	/**
	 * Rearrange the elements of an array in random order.
	 */
	public static void shuffle(Object[] a) {
		int N = a.length;
		for (int i = 0; i < N; i++) {
			int r = i + uniform(N - i); // between i and N-1
			Object temp = a[i];
			a[i] = a[r];
			a[r] = temp;
		}
	}

	/**
	 * Rearrange the elements of a double array in random order.
	 */
	public static void shuffle(double[] a) {
		int N = a.length;
		for (int i = 0; i < N; i++) {
			int r = i + uniform(N - i); // between i and N-1
			double temp = a[i];
			a[i] = a[r];
			a[r] = temp;
		}
	}

	/**
	 * Rearrange the elements of an int array in random order.
	 */
	public static void shuffle(int[] a) {
		int N = a.length;
		for (int i = 0; i < N; i++) {
			int r = i + uniform(N - i); // between i and N-1
			int temp = a[i];
			a[i] = a[r];
			a[r] = temp;
		}
	}

	/**
	 * Rearrange the elements of the subarray a[lo..hi] in random order.
	 */
	public static void shuffle(Object[] a, int lo, int hi) {
		if (lo < 0 || lo > hi || hi >= a.length)
			throw new RuntimeException("Illegal subarray range");
		for (int i = lo; i <= hi; i++) {
			int r = i + uniform(hi - i + 1); // between i and hi
			Object temp = a[i];
			a[i] = a[r];
			a[r] = temp;
		}
	}

	/**
	 * Rearrange the elements of the subarray a[lo..hi] in random order.
	 */
	public static void shuffle(double[] a, int lo, int hi) {
		if (lo < 0 || lo > hi || hi >= a.length)
			throw new RuntimeException("Illegal subarray range");
		for (int i = lo; i <= hi; i++) {
			int r = i + uniform(hi - i + 1); // between i and hi
			double temp = a[i];
			a[i] = a[r];
			a[r] = temp;
		}
	}

	/**
	 * Rearrange the elements of the subarray a[lo..hi] in random order.
	 */
	public static void shuffle(int[] a, int lo, int hi) {
		if (lo < 0 || lo > hi || hi >= a.length)
			throw new RuntimeException("Illegal subarray range");
		for (int i = lo; i <= hi; i++) {
			int r = i + uniform(hi - i + 1); // between i and hi
			int temp = a[i];
			a[i] = a[r];
			a[r] = temp;
		}
	}

	/**
	 * Unit test.
	 */
	public static void main(String[] args) {
		int N = Integer.parseInt(args[0]);
		if (args.length == 2)
			StdRandom.setSeed(Long.parseLong(args[1]));
		double[] t = { .5, .3, .1, .1 };

		StdOut.println("seed = " + StdRandom.getSeed());
		for (int i = 0; i < N; i++) {
			StdOut.printf("%2d ", uniform(100));
			StdOut.printf("%8.5f ", uniform(10.0, 99.0));
			StdOut.printf("%5b ", bernoulli(.5));
			StdOut.printf("%7.5f ", gaussian(9.0, .2));
			StdOut.printf("%2d ", discrete(t));
			StdOut.println();
		}

		String[] a = "A B C D E F G".split(" ");
		for (String s : a)
			StdOut.print(s + " ");
		StdOut.println();
	}

}